Optical device, method for producing the same and recording and/or reproducing apparatus employing the same

ABSTRACT

An optical device used for converging a light beam on a signal recording surface of an optical disc includes an optical lens for converging the light beam on a signal recording surface of the optical disc and a light barrier portion provided on a surface of the optical lens facing the optical disc. The light barrier portion includes a light transmitting aperture through which is transmitted the light beam converged by the optical lens. The light beam illuminated on the optical disc has its diameter controlled by this light transmitting aperture. The light radiated by a light source so as to be incident to the optical device is converged by the optical lens. The light converged by the optical lens is transmitted through the light transmitting aperture and illuminated on the signal recording surface of the optical disc. The diameter of the light beam illuminated on the signal recording surface of the optical disc is controlled by the light transmitting aperture, and thus the numerical aperture NA of the optical device is determined.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical device which is applied to arecording and/or reproducing apparatus for recording and reproducinginformation signals for an optical recording medium such as opticaldiscs, a method for producing the device, and a recording and/orreproducing apparatus employing the device.

2. Description of Related Art

There has been proposed an optical disc apparatus in which an opticalsystem is arranged on the side recording layer of an optical disc toachieve a high numerical aperture (NA) and hence the high densityrecording. This optical disc apparatus uses, as an objective lens usedin an optical pickup, an optical device having two lenses, such as isshown in Japanese Laying-Open Patent Publication H-10-123410, as lightconverging means.

Of the two lenses of the optical device, shown in this Publication, theone lying towards the optical disc is a so-called hemi-spherical lens.This lens towards the optical disc and the opposite side lens arehereinafter referred to as a forward lens and a rear lens, respectively.In this optical device, the numerical aperture (NA) is determined by adiaphragm which is a light transmitting aperture provided ahead of anincidence point of a light beam radiated from a light source on theoptical device. In this optical device, the light beam for signalrecording, incident on the optical disc, and the light beam forreproducing the information signals recorded on the optical disc, areconverged by the forward and rear lenses to fall on the opticalrecording layer of the optical disc.

Meanwhile, in an optical device, used in a conventional optical discapparatus, the light beam converged by the forward and rear lenses isilluminated on the optical recording layer of the optical disc. Theoptical device, made up of the forward and rear lenses, is designed tosuppress an eccentricity from the center axis of the light beam due totilt of the optical axis or to assembling errors to a smallest possiblevalue.

In an optical disc system, a high data transfer rate, a higher band fornot only the focusing servo and tracking servo but also an actuator, andreduction in weight of the actuator, are being sought. For reducing theweight of the actuator, it is indispensable to reduce the size and theweight of the optical device loaded on the actuator, such that a higherassembling accuracy is required of the optical device. If the opticaldevice, thus reduced in size and weight, is constructed so that thenumerical aperture (NA) is determined by the diaphragm, the slightestdeviation from the center of the optical axis of the light beamilluminated on the optical disc leads to marked variations of thediameter of the light beam illuminated on the optical recording layer.

In realizing the high density recording of the optical disc, reductionin the spot size of the light beam illuminated on the optical recordinglayer of the optical disc is a requirement. For reducing the spot sizeof the light beam, it becomes necessary to increase the numericalaperture (NA) of the objective lens used for converging the light beam.

Meanwhile, if desired to realize a large numerical aperture (NA) by asingle lens, a large refracting power is required. If the refractingpower is increased, the radius of curvature of the objective lensbecomes smaller, with the result that the position matching tolerance ofthe refractive surfaces becomes narrow. Consequently, a limit on theorder of 0.6 is set for the numerical aperture (NA) for a single lens.

With the lens set, made up of two lenses, that is the forward and rearlenses, as described in the aforementioned Publication, it is possibleto increase the numerical aperture (NA). With this lens set, it isnecessary to provide for a constant separation between first and secondlenses and for accurate orientation of the second lens with respect tothe first lens.

The separation between the first and second lenses and the setting ofthe orientation of the second lens with respect to the first lens arebased on the outer profile of the respective lenses. With the first andsecond lenses, produced by injection molding employing metal molds, theouter profile of the lens needs to be trimmed to high precision. Withthe molding by the metal molds, it is only possible to mold the lens toa limited degree of precision, whereas, with the position matching basedon the outer profile, it is difficult to achieve high precision positionmatching. If the first and second lenses, assembled together, are notpositioned accurately relative to each other, the second lens may beplaced at a separation different from the design separation, tilted oroffset with respect to the first lens. If such variation in theseparation, tilt or offset is produced, there is generated an aberrationexceeding an allowable range prescribed for a single lens, for example,0.4 rms.

Among the lenses used for an optical pickup, there are a plastic lensproduced on injection molding of a synthetic resin, a glass lensproduced on glass molding, and a glass lens formed on polishing. Withthese lenses, it is unexceptionally difficult to reduce the radius ofcurvature to render it difficult to produce a small diameter opticallens. In order to overcome these inconveniences, such a lens having acontinuously curved surface, formed by applying the manufacturing methodfor a Fresnel lens, has been proposed. However, it is still difficultwith this proposed method to produce a high precision lens.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an opticaldevice designed to suppress eccentricity of an incident light beam fromthe optical axis.

It is another object of the present invention to provide a recordingand/or reproducing apparatus in which it is possible to suppressvariations of the spot diameter of the light beam converged andilluminated on the optical recording layer of the optical disc.

It is still another object of the present invention to provide anoptical device made up of plural lenses that can be accuratelyposition-matched and put together, and a method for producing thedevice.

It is yet another object of the present invention to provide a methodfor producing an optical device of a small size and a large radius ofcurvature to a high accuracy.

An optical device for converging a light beam illuminated on an opticalrecording medium according to the present invention includes lightconverging means for converging the light beam illuminated on arecording layer of the optical recording medium, and a light barrierportion formed on a surface of the light converging means facing theoptical recording medium. The light barrier portion has a lighttransmitting aperture for permitting the light beam converged by thelight converging means to pass therethrough. The diameter of the lightbeam illuminated on the optical information recording medium iscontrolled by the light transmitting aperture.

An optical lens forming the optical device according to the presentinvention is formed by dry etching on one surface of a substrate formedof an optical material. A light barrier film as a light barrier portionis formed on the opposite surface of the substrate. This light barrierfilm is patterned by a photolithographic technique to form a lighttransmitting aperture for transmitting the light beam converged by theoptical lens therethrough.

The optical device of the present invention includes an optical lensformed by dry etching. The position matching markers are provided aroundthe optical lens, while the position matching markers are formed aroundoptical lenses when forming the optical lenses by dry etching.

The optical device according to the present invention is prepared byforming an optical lens by dry etching on one surface of the substrateformed of an optical material.

In the method for the preparation of the optical device according to thepresent invention, a mask material corresponding to the shape of anoptical lens is formed on a substrate formed of an optical material andsubsequently the mask material is deformed so that its surface area isreduced by heat treatment. An optical lens having a shape conforming tothe shape of the mask is transcribed by dry etching on the substrate toproduce a hemispherical lens. As the etching gas, a gas mixture of atleast one selected from the group of an oxygen gas, an Ar gas and a Hegas and a fluorinated carbon gas is used.

The recording and/or reproducing apparatus according to the presentinvention uses the above-described optical device as a portion of aoptical system.

Other objects, features and advantages of the present invention willbecome more apparent from reading the embodiments of the presentinvention as shown in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing an embodiment of an optical deviceaccording to the present invention.

FIGS. 2 to 8 are cross-sectional views showing a manufacturing processof an optical device according to the present invention, step-by-step,FIG. 2 showing a step of forming a layer of a mask material on asubstrate, FIG. 3 showing a mask forming step, FIG. 4 showing a maskdeforming step by heat treatment, FIG. 5 showing a lens forming step bydry etching, FIG. 6 showing a step of forming a light barrier layer,FIG. 7 showing a resist layer forming step and FIG. 8 showing a lighttransmitting aperture forming step.

FIG. 9 is a side view showing an embodiment of an optical pickupemploying the optical device according to the present invention.

FIG. 10 is a plan view showing a forward lens provided with a positionmatching marker forming another embodiment of the optical deviceaccording to the present invention.

FIG. 11 is a plan view showing a rear lens provided with a positionmatching marker.

FIG. 12 is a plan view showing the state in which the forward and rearlenses have been position-matched and put together using a positionmatching marker.

FIG. 13 is cross-sectional view showing the state of forming a positionmatching marker.

FIGS. 14 to 17 are cross-sectional views of a substrate showing thestate of forming a light transmitting aperture provided in the forwardlens in a substrate carrying an optical lens, with FIG. 14 showing alight barrier layer forming step, FIG. 15 showing a resist layer formingstep, FIG. 16 showing a resist layer patterning step and FIG. 17 showingthe state in which the layer of the mask material has been formed.

FIGS. 18 to 21 are cross-sectional views showing the manufacturingprocess for an optical device according to the present invention, withFIG. 18 showing a step of forming a layer of a mask material on asubstrate, FIG. 19 showing a mask forming step, FIG. 20 showing a maskdeforming step by heat treatment and FIG. 21 showing a lens forming stepby dry etching.

FIG. 22 is a perspective view showing surface properties of a lensformed by a dry etching method employing a mixture of an oxygen gas anda fluorinated carbon gas.

FIG. 23 is a perspective view showing surface properties of a lensformed by a dry etching method employing only a fluorinated carbon gas.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, an optical device, a manufacturing method forthe optical device and a recording and/or reproducing apparatusemploying the optical device, according to the present invention, arehereinafter explained in detail.

The optical device according to the present invention is used for anoptical pickup for a recording and/or reproducing apparatus employing anoptical recording medium, such as an optical disc, and includes a pluralnumber of optical lenses forming means for converging a light beamilluminated on the recording layer of the optical recording medium. Theoptical device includes a forward lens 1 and a rear lens 2 arranged sothat the optical axis thereof coincide with each other, as shown in FIG.1.

The forward lens 1 forming the optical device is a so-calledhemi-spherical lens having its surface facing the optical recordingmedium formed as a flat surface 1 acarrying a light barrier film 3operating as light barrier; In the light harrier film 3 is bored a lighttransmitting aperture 4 forming a diaphragm allowing the passage of alight beam converged by the forward lens 1 therethrough, so that thecenter of the light transmitting aperture 4 is subsequent coincidentwith the optical axis P₁ of the forward lens 1, in such a manner thatthe diameter of the light beam illuminated on the light recording layerof the optical information recording will be controlled by this lighttransmitting aperture 4.

The light beam transmitted through the optical device of the presentinvention and then illuminated on the optical recording layer has itsdiameter controlled by the light transmitting aperture 4 provided in thelight barrier film 3 to determine the numerical aperture of the opticaldevice.

Meanwhile, with a view to preventing unneeded reflection, the lightbarrier film 3 is preferably provided with antireflection film(s) by,for example, AR(Anti Reflection) coating. The antireflection film(s) arepreferably formed on both surfaces of the light barrier film 3.

The manufacturing method for the optical device according to the presentinvention, in particular the forward lens 1 having the light barrierfilm 3, is hereinafter explained.

The schematics of the manufacturing process for the optical device areshown in FIGS. 2 to 8.

The manufacturing process for the optical device is mainly composed ofthe following five steps:

-   (a) a step of placing a material as a mask material on a substrate,    that is a step of coating the mask material to a preset thickness    e.g., by a spin coating method, in case of using a photosensitive    material as the mask material;-   (b) a step of patterning the mask material (step of light exposure    and development in case of using a photosensitive material as the    mask material);-   (c) a step of deforming the mask material by heat treatment, such as    to reduce its surface area, for deforming the mask material to a    shape having a moderately curved surface;-   (d) forming a shape complying with the shape of the mask material in    an optical material (forming a shape complying with the shape of the    mask material in the optical material using a dry etching method in    the present embodiment); and-   (e) forming a light barrier film and forming a light transmitting    aperture therein.

FIG. 2 shows the above step (a). First, a photosensitive material iscoated by e.g., spin coating, on a substrate 11, formed of an opticalmaterial, to form a layer of a mask material 12.

The layer of a mask material 12 then is patterned by light exposure anddevelopment, to form masks 13 in register with respective lenses, asshown in FIG. 3.

The masks 13 are then heat-treated such as to reduce the surface area ofthe masks 13 to deform the masks 13 to an optically smooth curvedsurface, as shown in FIG. 4.

It should be noted that, if an optional photosensitive material is usedas a mask material, it does not necessarily occur that the mask materialbe deformed by heat treatment to reduce its surface area to give rise toa curved optically smooth surface.

For example, the present inventors conducted a trial operation with theheat treatment temperature ranging between 110° and 250° C., and foundthat, when the heating was carried out at a temperature not lower than200° C., so-called scorching, which produced transmutation in any resistmaterial, occurred unexceptionally. Such transmutation gives rise to anonuniform etching rate, so that, even if it is desired to produce ashape complying with the shape of the mask material, the shape producedtends to be disturbed.

From the experimental results, in order for the mask material to becomerounded on heat treatment to such a degree that an optically smoothsurface will thereby be produced, the glass transition temperature Tg ofthe mask material needs to be lower than the heat treatment temperature.Moreover, if desired to form the mask shape to a shape of an opticallens by e.g., dry etching, it is essential that the mask material be notsubjected to transmutation following heat treatment. So, the heattreatment temperature needs to be such a temperature which does notproduce transmutation in the mask material.

If a plating layer is formed on the mask material and a replica is to beformed using the plating layer as a mold, the above-mentioned conditionfor the heat treatment temperature is not necessarily required becauseof lack of the etching process for the mask material. However, eventhough a replica is to be formed, the surface of the mask material tendsto be roughed in case the mask material is transmuted on heat treatment.So, the condition of “the heat treatment temperature being such as notto cause transmutation of the mask material” is still a desirablecondition even in case of forming such replica.

Moreover, if the mask is deformed in the holding state of a substratecarrying a mask, the process reproducibility is lowered, whereas, if themask is deformed during the dry etching process, the processreproducibility is similarly lowered, so that it is necessary for theglass transition temperature Tg of the mask material to be higher thanthe holding temperature or the working process temperature. In theprocess of the present invention, the holding temperature is the roomtemperature, with the working process temperature being a temperature inthe vicinity of the room temperature.

Since the glass transition temperature Tg generally indicates a boundarytemperature for the state in which a material assumes a vitreous state,that is in which the material is fluidized without assuming a definitestructure, the heat treatment temperature is desirably a temperaturehigher than the glass transition temperature Tg by a sufficient margin,if process stability is taken into consideration. That is, for deformingthe mask material to reduce its surface area by heat treatment, the heattreatment temperature is desirably higher tens of degrees Centigradethan the glass transition temperature Tg. The deformation here enablesthe mask material to be fluidized by heat treatment to cause the maskmaterial to be deformed by its surface tension.

Specifically, by setting the heat treatment temperature so as to behigher by approximately 40° C. than the glass transition temperature Tg,the mask can be deformed to a round shape in less than one hour, thusassuring high efficiency production.

From the similar perspective, as for the relation between the glasstransition temperature Tg and the holding or process workingtemperature, the difference between the holding or process workingtemperature and the glass transition temperature Tg may be less thantens of degrees Centigrade.

As described above, the masks 13 are deformed to a round shape, afterwhich the shape corresponding to the shape of the masks 13 is formed inthe optical material. specifically, the shape corresponding to the shapeof the masks 13 is formed in the optical material, using a dry etchingmethod. This gives hemispherical lenses 14.

In the present embodiment, a molten quartz substrate is used as a glassmaterial forming the substrate 11, and a photosensitive material iscoated thereon to a thickness of approximately 20 μm, after which acircular pattern of approximately 120 μm is formed on light exposure anddevelopment. This circular pattern then is deformed at a heat treatmenttemperature of 150° C. to produce an optical lens by high density plasmaetching with the aid of the magnetic neutral loop discharging(high-speed etching employing NLD plasma).

With the NLD (magnetic neutral loop discharge) plasma etching device,high efficiency discharging can be produced to generate high densityplasma, by allowing the RF current to flow in a zero magnetic fluxdensity formed to a ring shape in vacuum, thus enabling high speedetching with a damage-free process.

In the preparation of micro-sized lenses of SiO₂, NLD plasma etching iscarried out, with the use of three different gases, C₂F₆, C₃F₈ and O₂ asetching gases in an atmosphere of the gas pressure in the etchingchamber of 0.27 Pa, with the antenna power of 900 W and with the biaspower of 300 W, to transcribe the shape of the photoresist to a quartzsubstrate.

The so formed optical lens has an optically smooth curved surface and,moreover, is a high NA optical lens of an extremely small diameter ofthe optical lens portion thereof on the order of 120 μm, with theoptical lens having a crest on the order of 30 μm.

Moreover, with the so produced optical lens, the contact positionbetween the substrate 11 and the mask 13 is not moved even after theheat treatment process, so that the mask 13 has its shape delimited bythe boundary line.

It should be noted that, since the boundary line of the mask 13 isprescribed by a photomask used in exposing the photosensitive materialto light, the optical lens is formed at an extremely finely definedposition. On the other hand, the optical lens has its height prescribedby the boundary line.

Since the optical lens, prepared by the above-described process, isprescribed by the photomask used in exposing the photosensitive materialto light, the position of the optical lens position as the sole lens andthe relative positions of the two lenses of the set in case of amulti-lens unit comprised of plural optical lenses formed on the samesubstrate or in case of a lens array may be defined to high precision.Moreover, the produced optical lens may find a wide field of applicationsince the optical lens produced may be of a larger NA than theconventional optical lens formed by a diffusion technique.

The optical lens then is adjusted to a preset thickness by a polishingstep. Subsequently, a light barrier layer 15 is formed on a surface of ametal, such as Cr, on the opposite side of the surface of the substrate11 of an optical material presenting the curved surface, as shown inFIG. 6. An AR coating is preferably applied to each surface of the lightbarrier layer 15 by way of eliminating stray light.

A resist layer 16 then is formed on the light barrier layer 15 andpatterned in register with the light transmitting aperture, as shown inFIG. 7. The light barrier layer 15 is then etched off to form the lighttransmitting aperture 4, as shown in FIG. 8.

The position of forming the light transmitting aperture 4 in the opticallens as light converging means is preferably adjusted using, forexample, the double-side photolithographic technique.

The optical device of the present invention, fabricated by the aboveprocess, may be used e.g., as an optical pickup of a recording and/orreproducing apparatus for recording and/or reproducing informationsignals on or from the optical recording medium.

FIG. 9 shows an illustrative structure of an optical pickup having abuilt-in optical device of the present invention.

This optical pickup uses the aforementioned optical device as anobjective lens 21.

With the optical pickup according to the present invention, the linearpolarized light, radiated from a semiconductor laser as a light sourceand formed into a parallel light beam by a collimator lens, istransmitted through a polarizing beam splitter (PBS) 22 and a λ/4 plate(quarter wave plate) 23 so as to be polarized to a circular polarizedlight. The light beam, polarized to circular polarized light, isconverged by the objective lens 21 to fall through a disc substrate 25on the signal recording surface of an optical disc 24.

The disc substrate 25 of the optical disc is a thin type substrate witha thickness on the order of 0.1 mm.

The objective lens 21 is combined from two lenses, as described abovewith reference to FIG. 1, and has a numerical aperture (NA) of 0.7 to0.95.

The light beam incident on the optical disc and reflected by its signalrecording surface, follows its former route to pass through the λ/4plate 23 so as to be converted into a linear polarized light beamrotated through 90° in its direction of linear polarization with respectto that of the forward route. This light beam is reflected by thepolarized beam splitter 22 and illuminated on a photodetector (PD) 28through a converging lens 26 and a multiple lens 27 so as to beconverted into an output electrical signal.

The multiple lens 27 of the optical pickup has a cylindrical lightincident surface and a concave light radiating surface. The multiplelens 27 affords astigmatism to the incident light beam to the incidentlight beam for enabling detection of the focusing error signals by theso-called astigmatic method.

A photodetector 28 is, for example, a six-segment photodiode, andoutputs detection signals for performing focusing adjustment andtracking adjustment by the astigmatic method and by the so-calledthree-beam method, respectively.

With the optical device of the present invention, in which the diameterof the light beam directed to the optical recording layer of the opticalrecording medium is limited by the light transmitting aperture, it ispossible to diminish the variations in the light beam diameter toapproximately the tolerance value even in case the optical axis of thelight beam illuminated on the light recording layer of the opticalrecording medium is offset from the center axis of the optical device.

Moreover, since the light transmitting aperture controlling thenumerical aperture (NA) of the optical device according to the presentinvention can be formed in a metal film formed as light barrier on thesurface of the optical lens, it can be formed readily. Moreover, sinceno mechanical supporting structure is needed, it is possible to diminishthe number of component parts.

Additionally, with the recording and/or reproducing apparatus of thepresent invention, in which the diameter of the light illuminated on thelight recording layer of the optical recording medium is limited by thelight transmitting aperture, it is possible to suppress variations inthe diameter of the light beam illuminated on the optical recordinglayer of the optical recording medium to a value corresponding to thetolerance value even in case the center of the light beam radiated fromthe light source is offset from the center axis of the optical componentdue to tilt of the optical axis or to assembling errors.

Referring to the drawings, a modification of the optical device of thepresent invention is explained in detail.

The optical device includes a forward lens 1 and a rear lens 2 arrangedso that the optical axis P₁ thereof coincide with each other, as shownin FIG. 1. In particular, the present modification is designed toprovide for facilitated assembling of the position-matched forward andrear lenses 1, 2.

In the optical device of the present embodiment, position matchingmarkers are provided around the forward lens 1 and the rear lens 2. Thelenses can be assembled with the respective optical axis coincident witheach other based on the position matching of these position matchingmarkers.

The forward lens 1 is provided with a cross-shaped position matchingmarker 33, shown in FIG. 10, whereas the rear lens 2 is provided with arecessed position matching marker 34 having slits 34 a registering withthe cross shape of the position matching marker 33.

If the forward lens 1 and the rear lens 2 are position-matched so thatthe markers 33, 34 register with each other, as shown in FIG. 12, usinge.g., a CCD camera, the forward lens 1 and the rear lens 2 areposition-matched to high accuracy as to separation, tilt or offset.

It should be noted that a small clearance is provided between the outershape of the position matching marker 33 formed in the forward lens 1and the slits 34 a of the position matching marker 34 provided in therear lens 2, this clearance representing the range of tolerance inposition matching.

If, after initial position matching, a certain relative mismatching inposition occurs between the forward lens 1 and the rear lens 2, positionmatching can again be achieved by exploiting the position matchingmarkers 33, 34.

As the position matching markers 33, 34, a crest and recess pattern,produced e.g., by etching, or a pattern obtained on etching e.g., ametal film, may be used. Alternatively, in the case of a lens, producedon molding, a mark may be provided on a metal mold and transcribed tothe lens for use as the position matching marker.

The position matching markers 33, 34 provided on the forward lens 1 andthe rear lens 2, are formed simultaneously at the process step offorming a hemispherical lens 14 corresponding to the hemispherical mask13 shown in FIG. 5 in the optical material in the course of thepreparation of the optical device as described above. Specifically, whenforming the hemispherical lens 14 corresponding to the hemisphericalmask 13 by the dry etching method, the position matching markers 33, 34are formed at the same time as the hemispherical lens 14 is formed, asshown in FIG. 13. The position matching markers 33, 34 are formed by aportion of the optical material forming the hemispherical lens 14.

In the optical device in which the forward lens 1 and the rear lens 2are provided with the position matching markers 33, 34 used forregistration, the position matching marker 33 may be used in forming thelight transmitting aperture 4 in the forward lens 1 to render itpossible to produce the forward lens 1 in which the optical axis of thelens is coincident with the center of the light transmitting aperture 4.This lens manufacturing method is now explained.

For fabricating this lens, a light barrier layer 15 is formed on thesurface of the substrate 11 of the optical material opposite to itssurface carrying the hemispherical lens 14 and the position matchingmarkers 33, 34, as shown in FIG. 14. A resist layer 16 then is formed onthe light barrier layer 15, as shown in FIG. 15, and the resist layer 16is patterned in register with the light transmitting aperture, using aphotomask 18, to allow the patterned resist layer 16 go be left over, asshown in FIG. 16. The light barrier layer 15 is removed by the etchingprocess to form the light transmitting aperture 4, as shown in FIG. 17.

The position of the light transmitting aperture 4 in the optical lens 14forming the light converging lens may be adjusted by position matchingthe position matching marker 33 formed around the optical lens 14 with aposition matching marker 19 on the photomask 18. That is, by positionmatching the position matching markers 33, 19, the optical axis of theoptical lens 14 forming the forward lens 1 can be brought intocoincidence precisely with the center of the light transmitting aperture4.

Similarly to the optical device, formed as described above, the opticaldevice can be used for e.g., an optical pickup of a recording and/orreproducing apparatus recording and/or reproducing information signal onor from the optical recording medium. The optical pickup employing thisoptical device is constructed similarly to that shown in FIG. 9 andhence is not explained specifically.

In the above-described embodiments, the hemispherical lens, forming theoptical device, is formed by the dry etching method. The method offorming the hemispherical lens on the substrate of the optical materialusing this dry etching method is now explained specifically.

The optical device in the present embodiment is formed by the processshown in FIGS. 18 to 21.

The manufacturing process for the optical device is mainly composed ofthe following five steps:

-   (a) a step of placing a material as a mask material on a substrate,    that is a step of coating the mask material to a preset thickness    e.g., by a spin coating method in case of using a photosensitive    material as the mask material;-   (b) a step of patterning the mask material (step of light exposure    and development in case of using a photosensitive material as the    mask material);-   (c) a step of deforming the mask material by heat treatment, such as    to reduce its surface area, for deforming the mask material to a    shape having a moderately curved surface;-   (d) forming a shape complying with the shape of the mask material in    an optical material (forming a shape complying with the shape of the    mask material in the optical material using a dry etching method in    the present embodiment).

The above step (a) is shown in FIG. 18. First, a photosensitive materialis coated by e.g., spin coating, on a substrate 51, formed of an opticalmaterial, to form a layer of a mask material 52.

The layer of a mask material 52 then is patterned by light exposure anddevelopment, to form masks 53 in register with respective lenses, asshown in FIG. 19.

The masks 53 are then heat-treated such as to reduce the surface area ofthe masks 53 to deform the masks 53 to a curved optically smoothsurface, as shown in FIG. 20.

The technique of deforming the mask 53 to an optically smooth surface issimilar to that described in the above embodiment and is not explainedspecifically.

The masks 53 are deformed to a round shape, as shown in FIG. 20, afterwhich a shape conforming to the shape of the masks 53 is formed in theoptical material forming the substrate 51. Specifically, the shapecorresponding to the shape of the masks 53 is formed in the opticalmaterial forming the substrate 51, using the dry etching method. Thisforms the hemispherical lens 54 forming the optical device of thepresent invention.

As the dry etching used for forming the hemispherical lens on theoptical material, a high density plasma etching employing the magneticneutral loop discharging (high-speed etching by the NLD plasma) isdesirably employed.

For dry etching, selection of etching gases is crucial. Among the usableetching gases, there is a gas mixture comprised of at least one selectedfrom an oxygen gas, an Ar gas and an He gas, and a fluorinated carbongas.

Among the fluorinated carbon gases, there are, for example, CHF₃, CH₂F₂,CF₄, C₂F₆, C₃H₈, C₄H₈ and C₆H₈.

Examples of specified combinations include C₃F₈+Ar, C₃F₈+O₂, C₃F₈+Ar+O₂,C₃F₈+He, C₃F₈+Ar+He, C₃F₈+He+O₂ and C₃F₈+Ar+He+O₂.

In the above etching gas, Ar has the effect of improving surfaceproperties by physical etching. The oxygen gas has the functions ofremoving carbonized products, and of adjusting a selection ratio. He hasthe functions of adjusting the gas pressure for maintaining thedischarge and of performing moderate physical etching. With the use ofany of the above-mentioned gas mixtures, such a lens having smoothsurface properties can been formed.

FIG. 22 shows surface properties of the hemispherical lens 54,fabricated using C₃F₈+C₂F₆+O₂ as the etching gas, whereas FIG. 23 showssurface properties of the hemispherical lens 54, fabricated usingC₃F₈+C₂F₆ as the etching gas.

It may be seen that use of the oxygen gas in combination leads to markedimprovement in the surface properties. With the use only of thefluorinated carbon gas, fine residues are left over on the surface.

Finally, the hemispherical lens 54 is adjusted to a preset thickness bythe polishing step to complete the optical lens forming the opticaldevice according to the present invention.

In the present embodiment, as in the previous embodiment, a moltenquartz substrate is used as the glass material of the substrate 51, anda photosensitive material is formed thereon to a thickness of 20 μm,after which a circular pattern of approximately 120 μm is formed by aprocess of light exposure and development. This pattern is deformed at aheat treatment temperature of 150° C. to produce an optical lens by highdensity plasma etching (high speed etching employing NLD plasma).

With the NLD (magnetic neutral loop discharge) plasma etching device,high efficiency discharging can be produced to generate high densityplasma, by allowing the RF current to flow in a zero magnetic fluxdensity formed to a ring shape in vacuum, thus enabling high speedetching with a damage-free process.

In the preparation of micro-sized lenses of SiO₂, NLD plasma etching iscarried out, with the use of three different gases, C₂F₆, C₃F₈ and O₂ asetching gases in an atmosphere of the gas pressure in the etchingchamber of 0.27 Pa, with the antenna power of 900 W and with the biaspower of 300 W, to transcribe the shape of the photoresist to a quartzsubstrate.

The so formed optical lens has an optically smooth curved surface and,moreover, is a high NA optical lens of an extremely small diameter ofthe optical lens portion thereof on the order of 120 μm, with theoptical lens having a crest with a height on the order of 30 μm.

Moreover, with the so produced optical lens, the contact positionbetween the substrate 51 and the mask 53 is not moved even after theheat treatment process, so that the mask 53 has its shape delimited bythe boundary line.

It should be noted that, since the boundary line of the mask 53 isprescribed by a photomask used in exposing the photosensitive materialto light, the optical lens is formed at an extremely finely definedposition. On the other hand, the optical lens has its height prescribedby the thickness of the mask 53.

Since the optical lens, prepared by the above-described process, isprescribed by the photomask used in exposing the photosensitive materialto light, the position of the optical lens position as the sole lens aswell as the relative positions of the two lenses of the set in case of amulti-lens unit comprised of plural optical lenses formed on the samesubstrate or in case of a lens array may be defined to high precision.Moreover, the produced optical lens has a wide field of applicationsince the optical lens produced may be of a larger NA than theconventional optical lens formed by a diffusion technique.

1. A method for preparing an optical device for converging a light beamilluminated on an optical recording medium, said method comprising thesteps of: forming light converging means on one surface of a substrateconsisted of an optical material by dry etching; forming a light barrierfilm on an opposite side surface of said substrate; and patterning saidlight barrier film by a photolithographic technique to form a lighttransmitting aperture for transmitting the light beam converged by saidlight converging means therethrough.
 2. The method for preparing theoptical device according to claim 1 wherein a position of forming saidlight transmitting aperture in said light converging means is adjustedusing a double-side photolithographic technique.
 3. The method forpreparing the optical device according to claim 1 further comprising thesteps of: forming a mask material in correspondence with a shape of anoptical lens on said substrate consisted of the optical material andsubsequently deforming the mask material by heat treatment so that asurface area of the substrate be reduced; and transcribing the lightconverging means shaped in compliance with the shape of said maskmaterial by dry etching on said substrate.
 4. The method for preparingthe optical device according to claim 3 wherein a temperature of theheat treatment is higher than a glass transition temperature of the maskmaterial and lower than a carbonizing temperature of the mask material.